The present invention relates to a semiconductor superlattice structure which is applicable to a light emitting device.
A light emitting device using a III-V compound semiconductor has been developed and has achieved emissions in the visible (red) to infrared wavelength range. The prerequisite for a light emitting material is that the energy gap Eg specific to the material is of a direct transition type. Where such a material is used to constitute an active (emission) layer of a light emitting diode, semiconductor laser or like device, there can be generated emissions with a wavelength .lambda..sub.g (nanometer)=1240/Eg (electron-volt) as is determined by the energy gap Eg. Usually, an active layer is provided by depositing a compound semiconductor, having an energy gap Eg which is associated with a desired emission wavelength .lambda..sub.g, to a thickness of several hundred angstroms or above. In the case where an energy gap associated with a desired wavelength cannot be implemented with GaAs or like compound semiconductor made up of two different elements, i.e., binary compound semiconductor, it is permissible to use a suitable mixed crystal consisting of three or four different elements and having a uniform composition, i.e. a ternary mixed crystal compound semiconductor such as Al.sub.x Ga.sub.1-x As or a quarternary mixed crystal compound semiconductor such as In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y.
A light emitting device of the kind providing an active layer as described above will hereinafter be referred to as an ordinary type light emitting device. In this case, the value which can be selected as the shortest emission wavelength is dependent on the largest one of direct transition type energy gaps, or direct energy gaps, Eg. So far as III-V compound semiconductors are concerned, the largest direct energy gap Eg is 2.3 electron-volts which is the energy gap of Al.sub.0.45 In.sub.0.55 P, a ternary mixed crystal compound semiconductor; a wavelength .lambda..sub.g which is associated with such an energy gap is nearly equal to 540 nanometers (H. C. Casey Jr. and M. B. Panish "Heterostructure Lasers; part B", Academic Press, New York, 1978, pp. 43-45). In practice, high quality layers of compound semiconductor crystals are unattainable unless they are deposited with their lattice constants substantially matched on a suitable single crystal substrate and, in this regard, Al.sub.x Ga.sub.y In.sub.1-x-y P of the type being deposited with matched lattice constants on a GaAs single crystal substrate is presently accepted as a material having the shortest emission wavelength which can be implemented with a III-V compound semiconductor. Based on such an idea, efforts have been made to shorten the emission wavelength of light emitting elements (H. Asahi, Y. Kawamura and H. Nagai, J. Appl. Phys. 53 (1982), pp. 4928-4391; Y. Kawamura, H. Asahi, H. Nagai and T. Ikegami, Electron. Lett. 19 (5) (1983) pp. 163-165). In this case, the largest value of direct energy gaps is 2.2 electron-volts and the shortest emission wavelength is substantially 560 nanometers (green).
Meanwhile, from an application standpoint, a light emitting element lying in a shorter wavelength range, i.e., blue-green to green range, is desired. However, meeting such a desire by use of a III-V compound is generally considered difficult for the previously stated reason. A light emitting material which emits in the blue-green to green range may be implemented by ZnSe or like II-VI compound semiconductor. Nevertheless, such has also been impracticable because many of constituent elements of a II-VI compound semiconductor, compared to those of a III-V compound semiconductor, have high vapor pressures so that a reduction of lattice defects which are introduced during the course of crystal growth is far more difficult in a II-VI compound semiconductor than in a III-V compound semiconductor. In this regard, shortening the emission wavelength using a III-V compound semiconductor whose lattice defects are relatively easy to control would be an invaluable contribution to the art.